The present invention relates to an electrophotographic developer and more particularly to a two-component electrophotographic developer containing a toner and a carrier, to be used for an image forming apparatus such as an electrostatic copying apparatus, a laser beam printer or the like.
In the image forming apparatus above-mentioned, the surface of a photoreceptor is exposed to light to form an electrostatic latent image on the surface of the photoreceptor. By a developing device, an electrophotographic developer is let come in contact with the surface of the photoreceptor. The toner contained in the electrophotographic developer is electrostatically sticked to the electrostatic latent image, so that the electrostatic latent image is formed into a toner image. From the photoreceptor surface, the toner image is transferred to and fixed on paper. Thus, an image corresponding to the electrostatic latent image is formed on the paper surface.
As the electrophotographic developer, there is generally used a two-component developer containing a toner and a carrier which is adapted to circulate in the developing device while adsorbing the toner.
As the toner, there may be used one as obtained by blending a fixing resin with a coloring agent such as carbon black, a charge controlling agent and the like and by pulverizing the blended body into particles having sizes in a predetermined range.
As the carrier, there may be preferably used a carrier having a core material made of iron particles or the like, of which surface is coated with a coating resin. The object of such coating of the carrier core material at the surface thereof with a coating resin is to control the toner electric charge amount and polarity, to improve the dependency of the developer electric charge on humidity and to prevent the occurrence of filming.
As the fixing resin and the coating resin, a styrene-acrylic copolymer may be suitably used in view of ease of handling and the like.
However, a conventional electrophotographic developer presents the following problems. That is, the electric charging characteristics are unstable at the initial stage just after a toner and a carrier are agitated and mixed with each other to cause the toner and carrier to be electrically charged at the time of the production of a start developer, the resupply of a toner or the like. Further, when a developing operation is repeated, the electric charging characteristics are deteriorated and become unstable with the passage of time. When the electric charging characteristics become unstable, the image density may vary and the image may present fog due to toner scattering. Further, if a great amount of toner scatters, the toner consumption is increased and the toner density of the developer varies, failing to maintain the proper toner density.
The following is considered to be two main reasons of why the electric charging characteristics of an electrophotographic developer become unstable.
(1) Variations in electric charging characteristics due to the composition of a styrene-acrylic copolymer used as a toner fixing resin or a carrier coating resin PA1 (2) Variations in electric charging characteristics due to an electric charge controlling dye contained in a toner fixing resin for adjusting the electric charging characteristics
First, the following will discuss the variations in electric charging characteristics due to the composition of a styrene-acrylic copolymer.
The inventors of the present invention have studied, from various points of view, the relationship between the electric charging characteristics of an electrophotographic developer and the composition of a styrene-acrylic copolymer, i.e., the types and blending ratio of a styrene component and an acrylic component contained in the styrene-acrylic copolymer. The inventors have found that the electric charging characteristics of the electrophotographic developer depend on the type and amount of a functional group (such as a --COO-- group or the like) of the acrylic component in the styrene-acrylic copolymer.
It has been known that the functions of the developer such as mechanical properties, paper-adhesive properties of the toner, off-set and the like depend on the blending ratio of the styrene component and the acrylic component in a styrene-acrylic copolymer. In this connection, the blending ratio of the styrene component and the acrylic component in each of the toner fixing resin and the carrier coating resin has been determined in view of the mechanical properties and the like above-mentioned.
It has not been known, however, the electric charging characteristics of an electrophotographic developer have depended on the type and amount of a functional group of the acrylic component in a styrene-acrylic copolymer. Accordingly, no consideration has been made at all on such data and there has been used a styrene-acrylic copolymer in which the type and amount of a functional group of the acrylic component have not been properly determined, so that the developer has showed variations in electric charging characteristics.
To eliminate variations in electric charging characteristics due to the composition of a styrene-acrylic copolymer to stabilize the electric charging characteristics without injury to the mechanical properties and the like, the inventors have tried to determine a range of the content of an acrylic component in a toner fixing resin, a range of the content of an acrylic component in a carrier coating resin and a range of the ratio of both contents above-mentioned. However, the inventors could not stabilize the electric charging characteristics perfectly only with the determination of the ranges above-mentioned.
Now, the following description will discuss the variations in electric charging characteristics due to an electric charge controlling dye for adjusting the electric charging characteristics.
In a conventional electrophotographic developer, an electric charge controlling dye has been contained in a toner fixing resin in order to prevent the electric charging characteristics from varying as above-mentioned.
However, the conventional developer has presented a problem that, even though the content of the electric charge controlling dye has been constant, the toner has shown variations in electric charging characteristics, causing the developer to become unstable in electric charging characteristics.
According to the study of the inventors, it has been found that the electric charging characteristics of a toner have been determined by a surface dye density, i.e., the amount of an electric charge controlling dye which has been exposed to the surfaces of the toner particles to contribute to the transfer of electric charge. Conventionally, the dispersion of the electric charge controlling dye in the toner particles has not been uniform so that, even though the content of the electric charge controlling dye has been the same, the surface dye density has not been constant, causing the electric charging characteristics of the toner to vary.
In view of the foregoing, the inventors have tried to stabilize the electric charging characteristics of an electrophotographic developer by determining a preferable range of the surface dye density of the toner particles to eliminate variations in electric charging characteristics of the toner. However, only with the determination of the range of the surface dye density, the electric charging characteristics could not been perfectly stabilized.